Studies designed to provide insight into the specific substances and regulatory mechanisms that underlie the tumorous state in plants and on the reversal of that state have continued during the past year. A cAMP (3'5')-like compound was isolated and purified from both normal and tumor cells of plants. Although this substance has certain properties in common with authentic cAMP (3'5') and is a nucleotide-size compound, it differs strikingly from that compound in a number of essential respects. In contrast to authentic cAMP (3'5') this compound has been found to be a very potent inhibitor of both beef heart cyclic monophosphate phosphodiesterases and of a cAMP-dependent protein kinase. The role played by this substance in the regulation of cell division in higher plant species was further investigated. A second area of the plant tumor problem that was studied was concerned with the persistent but potentially reversible suppression of the tumorous state in crown gall teratoma tissues of tobacco. The differential expression of the Ti plasmid-specified functions, namely oncogenicity and nopaline synthesis, were investigated under conditions in which the tumorous state was suppressed. It was found that while nopaline is synthesized persistently, a reexpression of oncogenicity depends on the development of new cell divisions. These new cell divisions appear to be required either to reestablish positive feedback control mechanisms upon which the development of autonomy in plant tumor cells appears to depend, or they are required to establish the pattern of metabolism concerned with cell growth and division which is then maintained in the plant tumor cells by positive feedback control mechanisms.